Technical Field
The invention relates to the field of inkjet printing. More specifically, the invention relates to techniques for more efficient curing of ultraviolet curable ink deposited in a printing environment.
Description of the Related Art
Ultraviolet curing of liquid chemical formulations has been an established practice for many years. In ultraviolet curing, a liquid chemical formulation comprising photoinitiators, monomers and oligomers, and possibly pigments and other additives is exposed to ultraviolet light, thereby converting the liquid chemical formulation into a solid state.
Ultraviolet-curable inks are oftentimes used advantageously in the field of ultraviolet inkjet printing. In these applications, ultraviolet-curable ink is jetted from a print head onto a substrate to form a portion of an image. Typically the print head scans back and forth across a width of the substrate, while the substrate steps forward for progressive scan passes. Thus a relatively small print head is used to build a very large image.
In some cases of ultraviolet inkjet printing, an ultraviolet light source is mounted on either side of a print head to cure the ink. Using this configuration, ultraviolet-curable ink can be jetted and cured in the same print head pass. Other times, the ink is jetted in one pass and cured in a subsequent print head pass.
In some cases of ultraviolet inkjet printing, the width of the print head is at least equal to that of the substrate and the entire image is formed with a single pass of the substrate underneath the print head. In these cases, the ultraviolet light source is typically in a fixed location, with the substrate moving under the print head first and subsequently under the ultraviolet light source.
As explained above, curing ink involves directing photons, typically with wavelengths in the ultraviolet spectrum, onto an ink deposit. The photons interact with photoinitiators present within the ink, creating free radicals. The created free radicals initiate and propagate polymerization (cure) of the monomers and oligomers within the ink. This chain reaction results in the ink curing to a polymer solid. However, the presence of oxygen at the ink surface inhibits such a chain reaction from occurring within the ink. This is often referred to as oxygen inhibition.
In normal ultraviolet curing in an air environment, a high amount of ultraviolet energy and/or a high concentration of photoinitiator are needed to achieve a full cure, compared to the ultraviolet power and photoinitiator concentration required in an oxygen free curing environment. Indeed, both higher ultraviolet energy and higher photoinitiator concentration deleteriously affect the final film properties, and increase ink and printer costs.
Common solutions for providing less reactive curing include completely supplanting atmospheric oxygen with a less reactive gas such as nitrogen in the cure zone. For example, U.S. Pat. No. 6,126,095 to Matheson et al., entitled “Ultraviolet Curing Apparatus Using an Inert Atmosphere Chamber” teaches a curing apparatus comprising a curing chamber for accommodating a controlled atmosphere. The curing chamber includes inlets and nozzle assemblies for supplying less reactive gas into the chamber and maintaining a less reactive atmosphere therein.
Likewise, U.S. Pat. No. 7,431,897 to Hahne et al., entitled “Apparatus Replacing Atmospheric Oxygen with an Inert Gas from a Laminar Air Boundary Layer and Application of Said Apparatus” (hereinafter referred to as “Hahne”) teaches completely replacing atmospheric oxygen with a less reactive gas.
These prior art references disclose specialized and expensive approaches to providing reduced oxygen curing conditions, but fall short of achieving feasibility for common inkjet printing systems, because it remains difficult and expensive to supply the printing environments with enough inerting gas to effectively rid the curing region of oxygen. It would be desirable to address this shortcoming.
Additionally, ultraviolet ink has a significant cost associated with it. Therefore, thicker films of ultraviolet-curable ink increase the cost of the finished image. It is oftentimes desirable to lay down as thin a film of ink as possible without compromising color strength. In typical ultraviolet inkjet printing applications, there is a small time delay before a jetted droplet of ink is exposed to the ultraviolet light source. In that time delay, sometimes known as “time to lamp,” the drop generally tends to spread out and wet the media. This phenomenon is known as “dot gain.” Longer time to lamp results in higher dot gain and thinner final ink layer thickness. However, longer times to lamp also tend to increase the size of the print head or printer, and decrease the overall print speed. It would be desirable to address this problem as well.
In scanning printer applications, droplets of ink are laid down then cured. Then additional drops are jetted onto the cured drops. This method tends to create a coarse surface finish, i.e. a matte finish, which reduces the glossiness of the image. In many applications, a high gloss finish is desirable in the final appearance of the print job. However, in some applications, it may be desirable to vary the level of gloss/matte or surface appearance. Current inkjet printing does not allow for such variations in surface appearance. It would be desirable to address these issues as well.